With all the many plot lines going on might Sandra Abe be someone that's been overlooked. Her job gives her access to the station's files which tells me she knows where all he bodies are buried. Upon approaching her Daniel immediately found Frost, while even Kirsch had no idea where he was. Miss Krug seems to see her as someone of value and tried questioning her unsuccessfully. She might very well be more deeply involved in the some of the shenanigans than we realize.
I've suspected Sandra of something or other for while now.
She has a good cover, first getting romantically involved with Frost and then due to that, having a reason to follow him to Berlin after their last posting together.
She seems to follow where he goes.
Romantic possibly? But perhaps something more sinister afoot. Also, she is in a support position at the office, so is not perhaps the first person who might be suspected-- a la Martha in "The Americans."
Someone in DC where I worked in the trenches for decades told me many years ago that secretaries and admin assistants in DC were the people intel gatherers often courted, because they knew they knew SO much. And were not high up enough to first be suspected to be targets for folks seeking information about an operation.
That's why the Martha character in "The Americans" rang so true to me, after all those years hearing of situations like that.
Hector has been giving his cross-dresser friend thumb drives which he leaves instructions for the reporter to pick up. Then the reporter goes public with it.
Most administration staff of diplomatic corps and intelligence services tend to be shifted every two or so years according to a friend who has been with the State Department for her entire career. This is precisely to avoid the situations you describe. The military does the same thing with few exceptions - like special forces, where small teams stay together for years to maintain unit cohesion - to prevent complacency; a blurring of loyalties as relationships grow between co-workers and foreign nationals; and access to too many secrets of one office or post. Of course, the political appointees tr ry to screw things up and the very nature of bureaucracy often render these efforts ineffective. Seniority plays a role as well.